It appears white because it is interacting with Luke's Force sensitivity. Since Luke is unusually powerful with the Force, yet unattuned to it like the Jedi of old, the lightsaber responded to his pureness with the Force by turning white- the color that contains red, green, and blue within its spectrum. As the trilogy displays, Luke possessed some dark side tendencies, and was on the threshold of the dark side more than once. He used elements of red-saber wielders and blue-/green-saber wielders. The white lightsaber is indicative of Luke's journey as a Jedi-to-be.

Similarly, Vader's saber appears lighter (as he is walking towards the blast door), but not quite white. Having just defeated Obi-Wan, the waning intensity of the red saber mirrors his state of mind as he contemplated Obi-Wan's words and behavior, and his possibly his past. Destroying Obi-Wan possibly triggered some deeply buried emotion, always Anakin's weak point, as he looked upon those who had been traveling with his former Master. Within him, as Luke would sense later on, there was good. The paling of his saber forehshadows his eventual return to the light side.

I think there is some reputable EU stuff that says lightsabers can interact with Force-sensitives. Something about Sith getting stronger by using the red crystal or something like that. Anyway, from the databank: "To a Jedi, a lightsaber is not just a weapon. It is a means of concentrating attention and becoming attuned with the Force." Obviously I was wildly extrapolating. "Moodsaber" or "mindsaber," yeah kinda, in metaphorical terms.

It's old special effects people. I'm beating they will fix them in the DVD's. You just wait.

.. or maybe Lucas wanted them white-ish looking. Heck I don't know, but I would suppose that in the PT they are vivid, bright neon-ish for show and Lucas will make up his mind to do more work on the OT by making the sabres look similar to the PT.